This research has elucidated the structure of the cloned 5.2-kilobase (kb), integrated DNA provirus of MH2; a major portion (3.5 kb) of the 3' region has been sequenced. The complete genetic structure has been determined and found to be: 5'-Delta gag(1.9 kb)-mht(1.2 kb)-myc(1.3 kb)-[non-coding] c-region(0.2 kb)-3'. Aside from the genetic elements shared in common with other nondefective avian retroviruses, the mht gene was found to be a unique oncogene sequence. Hybridizations have revealed that mht is derived from a normal cellular gene distinct from the proto-myc family of oncogenes. The Deltagag-mht forms a hybrid gene, containing a 2682 nucleotide contiguous reading frame with a stop codon near the 3' end. The 3' region of mht (containing 969 nucleotides) is 90% sequence-related to the one-specific raf sequence of the MSV 3611, and 95% homologous at the deduced amino acid sequence level; the closest homology determined so far of the 19 known onc sequences. Using a unique, expression vector, pJL6, we have cloned and expressed the carboxyterminal portion of the avian MC29, v-myc oncogene as a fusion protein in bacteria produced as more than 10% of the total cellular protein. This expression product has enabled the preparation of antibodies raised against it; these antibodies immunoprecipitate the MC29 oncogene product, p110gag-myc. Employing the same expression system, we prepared a Harvey MSV, p21ras oncogene product as a fusion protein, produced as more than 10% of the total protein; this protein exhibited GDP binding activity and was capable of autophosphorylation, similar to authentic, Ha-MSV p21ras. We have sequenced and characterized two reciprocal recombinant sites between c-myc and the immunoglobulin heavy chain IHC Mu region in a Burkitt lymphoma, showing that the onc gene is interrupted within its first intron region and joined to the IHC Mu switch region.